Fuel cells generally use hydrogen-rich gas (hereinafter referred to as “hydrogen-containing gas”) as a fuel. The hydrogen-containing gas is mainly generated through a steam reforming reaction between hydrocarbon-based material gas and steam. A typical reforming system (hydrogen generator) for generating the hydrogen-containing gas is composed of a reformer, a shift converter and a selective oxidizer. Since the steam reforming reaction is encouraged by a catalytic action, the reforming system includes a combustor such as a burner and is designed to heat the material gas etc. to a catalytic temperature while generating steam.
A typical fuel cell system is composed of a material gas feeder, a reformer, a fuel cell and a combustor which are aligned in order so as to be communicated with one another by means of a communication passage. Thereby, the material gas is converted into a hydrogen-containing gas that is, in turn, used in an electrochemical reaction in the fuel cell and excessive hydrogen-containing gas discharged from the fuel cell is efficiently used as a fuel for the combustor.
When shutting down the fuel cell, an inert gas or the like is allowed to flow within the fuel cell system. With this shutdown process, the hydrogen-containing gas remaining in the fuel cell system can be purged (purging treatment). As a typical purging treatment, a method for supplying an inert gas (e.g., nitrogen) to the reformer is generally taken. There has been proposed another purging treatment (e.g., Patent Document 1) according to which the material gas (e.g., city gas) used for generating a hydrogen-containing gas is supplied to the reformer etc. of the fuel cell system, the hydrogen-containing gas within the fuel cell system is purged by the material gas, and then the fuel cell system is sealed with the material gas.
Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2003-229156